This invention relates to a heat-sensitive recording material. More particularly, this invention relates to an improved heat-sensitive recording material in which a color reaction of an electron donating, colorless or pale-colored dye precursor (hereinafter referred to as "coupler") with an electron accepting compound (hereinafter referred to as "developer") is utilized.
A heat-sensitive recording material containing a binary coloring agent consisting of a coupler and a developer has the advantage that it can be printed in one step and easily handled. Therefore, it is mainly used as the heat-sensitive recording material.
The heat-sensitive recording material is widely used in such fields as facsimiles, recording in measuring machines, labels and the like. Particularly in the facsimiles, the demand for the heat-sensitive material has remarkably increased and excellent properties are required as stated below.
Since a high speed facsimile was developed, it has been required that a sufficient optical density be obtained even when printing is carried out with electricity having a narrow pulse width, namely, with a low energy, and the heat-sensitive recording material be not colored with the heat remaining in the thermal head after the printing. Furthermore, molten matters have had to be prevented as much as possible from adhering to the thermal head.
In order to satisfy the above contradictory requirements, for example, Japanese Patent Application Kokai No. 54-23545 proposes a heat-sensitive recording material comprising inorganic powder having an oil absorption of at least 50 ml/100 g. Japanese Patent Application Kokai No. 59-225987 proposes a heat-sensitive recording material comprising a layer containing an expanded plastic filler.
However, in the case of these heat-sensitive recording materials, which can be colored with a low energy, there is a tendency that the amount of the molten matters adhered to the thermal head increases. Therefore, when printing is effected for a long period of time, the printed image becomes unclear or the printing sometimes becomes impossible on account of the molten matters adhering to the thermal head. Even if substantially no molten matters adhere to the thermal head, the optical optical density obtained is remarkably low because the molten matters soak under the heat-sensitive coloring layer when the printing is carried out with high energy.